Trade-offs among growth performance, nutrient digestion and carcass traits when feeding low protein and/or high neutral-detergent fiber diets to growing-finishing pigs

• Low CP diets affected more the growth performance than high aNDFom diets.
• The CTTAD of CP was reduced by low CP diets but not by high aNDFom diets.
• The CTTAD of aNDFom was not reduced by high aNDFom and low CP feed.
• The slurry pH was reduced by decreasing dietary CP but not aNDFom.
• Back-fat thickness at slaughter was highest in pigs fed low CP or high aNDFom diets.

This study evaluated the effects of reducing dietary crude protein (CP) and increasing neutral detergent fiber (aNDFom) on growth performance, nutrient digestibility, manure composition and carcass parameters of lean pigs as a means of reducing the environmental load of slurry. Sixty-four intact male Landrace × Large-White pigs (13.8, SD 2.3 kg of initial body weight (BW)) were assigned to one of two dietary CP levels (high, HP or low, LP) and one of two aNDFom levels (high, HF or normal, NF) in a 2 × 2 factorial design, and subjected to a three-phase feeding program from 6 to 21 weeks of age (15–110 kg of BW). The diets had similar metabolizable energy (ME), total lysine content and ideal amino acids (AA) ratio. Pigs fed HP diets had the highest average daily gain (ADG) and BW from 12 weeks of age (P < 0.05), which was associated with a gain:feed ratio that was higher than in the LP treatment (P < 0.05). Dietary aNDFom did not affect the ADG or gain:feed of pigs (P > 0.05). The coefficient of total tract apparent digestibility (CTTAD) of CP was higher in HP groups (0.765 ± 0.0075), than it was in the LP groups (0.732 ± 0.0075, respectively), independent of the dietary aNDFom level. Low dietary CP reduced aNDFom digestibility in pigs fed diets that had a normal aNDFom level (LP-NF: 0.45), but not in pigs that were fed a high aNDFom diet (LP-HF: 0.548), compared to pigs fed HP diets (HP-NF: 0.546, and HP-HF: 0.583 ± 0.011). Low dietary CP increased the manure output at 21 week of age (P < 0.001) and high dietary aNDFom increased manure output from 16 weeks of age (P < 0.001). The slurry pH was higher in the HP groups than it was in the LP groups (7.42 vs. 7.18 ± 0.085, P = 0.05), but the level of dietary aNDFom did not alter the pH of slurry (P = 0.66). Backfat thickness at slaughter was highest in pigs fed low CP (P < 0.001) or high aNDFom diets (P = 0.03). In summary, the effects of dietary CP and aNDFom on growth performance were largely independent of each other. From 12 to 21 weeks of age, lowering dietary CP up to 50 g/kg reduced growth performance and increased back-fat at slaughter. High aNDFom diets did not reduce growth performance, but hampered lean tissue gain. An increase in dietary aNDFom in low CP feed did not reduce the CTTAD of aNDFom in growing-finishing pigs. However, this dietary manipulation had the lowest economic return.